How do I fill in missing faces?
There's multiple ways so let me explain them to you because things like these can really speed up your workflow

You can select any number of vertices (at least 3) and press 'F' to create a face between these vertices.

As you have probably already figured out, quads are the faces you want to go for so ideally you want to select 4 vertices to create a face.

Alternatively you can select two opposing edges to speed things up.

In your example you have like a row of unfilled faces, so you can just select the top edge of the last face before the missing face and press 'F'. Blender recognizes the pattern based on the surrounding faces and fills the next one it. This only works if you have one row of missing faces (in other words: have faces on 3 of 4 sides).

Another technique that's really handy is the grid fill. I think there's no shortcut for this out of the box so just hit F3 to open the search bar and type in &quot;Grid fill&quot;. This is especially helpful if you need to connect multiple faces that would result in a grid. An example: a cube that doesn't have a top face and is subdivided 3 times on the sides. Select the edge loop of the top and use grid fill to create top faces based on the subdivions of the sides.


Hope that helps, happy blending!